Plant biology Flashcards
Describe Transpiration
Transpiration is the loss of water vapour from the stems and leaves of plants
Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls
New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
How is water loss regulated GENERALLY?
The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata
-Guard cells flank the stomata and can occlude the opening by becoming increasingly flaccid in response to cellular signals
-When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
-Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
-A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
What factors affect transpiration rate?
Transpiration rates will be higher when stomatal pores are open than when they are closed
Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration
Other factors that will affect transpiration rates include humidity, temperature, light intensity and wind
What factors affect transpiration rate?
Transpiration rates will be higher when stomatal pores are open than when they are closed
Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration
Other factors that will affect transpiration rates include humidity, temperature, light intensity and wind
What properties of water facilitate the transpiration stream?
Cohesion:
Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)
Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream
Adhesion:
Adhesion is the force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
The xylem wall is also polar and hence can form intermolecular associations with water molecules
As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension
State the function and structure of Xylem
The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant
It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the free movement of water
Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only
The cell wall contains numerous pores (called pits), which enables water to be transferred between cells
Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension
Xylems can be composed of tracheids (all vascular plants) and vessel elements (certain vascular plants only)
Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer
In vessel elements, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer
All xylem vessels are reinforced by lignin, which may be deposited in different ways:
In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other
In spiral vessels, the lignin is present in the form of a helix or coil
What are the different root structures?
Some plants have a fibrous, highly branching root system which increases the surface area available for absorption
Other plants have a main tap root with lateral branches, which can penetrate the soil to access deeper reservoirs of water
How is water taken up by the root?
Water will follow the mineral ions into the root via osmosis – moving towards the region with a higher solute concentration
The rate of water uptake will be regulated by specialised water channels (aquaporins) on the root cell membrane
Once inside the root, water will move towards the xylem either via the cytoplasm (symplastic) or via the cell wall (apoplastic)
In the symplastic pathway, water moves continuously through the cytoplasm of cells (connected via plasmodesmata)
In the apoplastic pathway, water cannot cross the Casparian strip and is transferred to the cytoplasm of the endodermis
How may ions be taken up by the roots?
Mineral ions may passively diffuse into the roots, but will more commonly be actively uploaded by indirect active transport
-Root cells contain proton pumps that actively expel H+ ions (stored in the vacuole of root cells) into the surrounding soil
-The H+ ions displace the positively charged mineral ions from the clay, allowing them to diffuse into the root along a gradient
-Negatively charged mineral ions (anions) may bind to the H+ ions and be reabsorbed along with the proton
Describe the structure of the root
The epidermis of roots may have cellular extensions called root hairs, which further increases the surface area for absorption
Materials absorbed by the root epidermis diffuse across the cortex towards a central stele, where the xylem is located
The stele is surrounded by an endodermis layer that is impermeable to the passive flow of water and ions (Casparian strip)
How do Xerophytes conserve water?
Xerophytes
Xerophytes are plants that can tolerate dry conditions (such as deserts) due to the presence of a number of adaptations:
Reduced leaves – reducing the total number and size of leaves will reduce the surface area available for water loss
Rolled leaves – rolling up leaves reduces the exposure of stomata to the air and hence reduces evaporative water loss
Thick, waxy cuticle – having leaves covered by a thickened cuticle prevents water loss from the leaf surface
Stomata in pits – having stomata in pits, surrounded by hairs, traps water vapour and hence reduces transpiration
Low growth – low growing plants are less exposed to wind and more likely to be shaded, reducing water loss
CAM physiology – plants with CAM physiology open their stomata at night, reducing water loss via evaporation
How do Halophytes conserve water?
Halophytes
Halophytes are plants that can tolerate salty conditions (such as marshlands) due to the presence of a number of adaptations:
Cellular sequestration – halophytes can sequester toxic ions and salts within the cell wall or vacuoles
Tissue partitioning – plants may concentrate salts in particular leaves, which then drop off (abscission)
Root level exclusion – plant roots may be structured to exclude ~95% of the salt in soil solutions
Salt excretion – certain parts of the plant (e.g. stem) may contain salt glands which actively eliminate salt
Altered flowering schedule – halophytes may flower at specific times (e.g. rainy seasons) to minimise salt exposure
Describe two practical models of xylem
Capillary Tubing:
Water has the capacity to flow along narrow spaces in opposition to external forces like gravity (capillary action)
This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of the tube surface
The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are thin: 20 – 200 µm)
Filter Paper:
Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties
When placed perpendicular to a water source, the water will hence rise up along the length of the paper
This is comparable to the movement of water up a xylem (the paper and the xylem wall are both composed of cellulose)
Define translocation
Translocation is the movement of organic compounds (e.g. sugars, amino acids) from sources to sinks
The source is where the organic compounds are synthesised – this is the photosynthetic tissues (leaves)
The sink is where the compounds are delivered to for use or storage – this includes roots, fruits and seeds
In roots and stem how are xylem and phloem aranged?
Xylem always on the inside!
Xylem an X shape in Dicot root
Explain how the structure of the phloem is adpated to its function
Sieve Element Cells
Sieve elements are long and narrow cells that are connected together to form the sieve tube
Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells
Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials
The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow
Companion Cells
Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube
Describe how loading occurs
Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells
Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)
Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)
Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure
-Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)
-The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient
-Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement
-This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source
